In conventional luminous marking techniques, a metal or plastic segment of an object to be marked is provided with a small indentation or is countersunk. Into an indentation is placed a small quantity of a phosphorescent material and a source of nuclear radiation. In order to meet requirements imposed by the law, a protective cover must be provided to prevent direct contact with the radioactive material. A standard practice has been to use a plastic material as the cover. Also, it has been conventional to make the indentation containing the radioactive material sufficiently deep so that the exterior of the cover lies at a lower level than the surface of the surrounding object. Because the indentation and the cover are generally of small diameter, say about 2 mm, direct contact with the cover is not apt to occur. However, it has been found that if the object having one or more luminous markings is handled roughly or is used outdoors as is the case with a weapon, or if the object is exposed to abrasive materials, then a plastic cover may be damaged.
A more serious problem arises from the fact that the temperature coefficient of expansion of plastics is much larger than that of metals. Consequently, if the object is subjected to large changes in temperature as may be the case with a weapon which is fired many times in quick succession, then the plastic cover may fail to maintain effective closure over the indentation, eventually permitting escape of nuclear radiation.